In the past few years, there has been a tremendous increase in research activities on organocatalysis, especially chiral organocatalysis. Likewise, there is also a lot of interest in heterogeneous organocatalysis. However, supported organocatalysts are much less successful than heterogeneous organometallic catalysts partly because the organocatalysts are more sensitive to the chemical environment of the solid support.
Although a range of chiral ligands and their transition metal complexes have been developed and many of them are known to be highly effective in various asymmetric transformations, most chiral synthons are still produced from natural chiral building blocks or through resolution of racemic mixtures. Separation and recycling of expensive chiral catalysts are a major challenge preventing homogeneous asymmetric catalysts from being adopted more widely in industrial processes. However, this problem may be solved by converting homogeneous catalysts to heterogeneous catalysts through catalyst immobilization on an insoluble solid supports. For example, a pyrrolidine-based catalytic system has been well developed, and was successfully applied to a significant number of asymmetric transformations. However, developing a highly efficient heterogenized pyrrolidine organocatalysts remained a significant challenge.
Polymers and porous silica are commonly used as catalyst supports. A problem with heterogeneous catalysis is diffusion limitation: mass transfer (e.g. transfer of reagents) from the liquid phase to the active sites on the solid surface is a significant rate limiting process for the reaction. Mesoporous silica has superior physical properties: it has a robust, porous structure with high surface areas. For example, mesocellular foam has interconnected ultralarge pores (up to 30 nm) that further reduce the diffusion limitation problem. It is also possible to derive mesoporous silica in uniform, spherical particle morphology, facilitating the use of such material in packed bed reactors for continuous flow type processes. However, silica supports have surface silanol groups that tend to interact strongly with catalytic complexes. Thus, catalysts frequently suffer from decrease in catalyst activity and/or selectivity after immobilization on silica supports, and would require a multi-step catalyst immobilization process. Polymer supports do not have these problems, but are hitherto not characterized by well-defined, ultralarge pores. Diffusion limitation and polymer swelling remain as disadvantages despite the development of large-pore resins, low-swelling polymers, and mesoporous polymers.